The present invention relates to a method and apparatus for determining the zeta potential of particles in a suspension comprising, a cell containing at least a portion of the suspension of particles; a first electrode and a second electrode disposed in the cell; means for producing an electric field between the first and second electrodes, the electric field causing the particles to accelerate; means for converting a force, the force being generated by the acceleration of the particles, into electrical energy; and means for measuring the electrical energy. In particular, the present invention provides a novel method and apparatus for determining the zeta potential of colloidal particles by means of measuring the force created when an unsteady driving voltage is applied to the electrodes of a cell, the mechanical force produced by acceleration of the particles is converted to an electrical energy or signal by a piezoelectric mechanical resonator and the electrical energy being detected by a synchronous demodulator.
Determination of the zeta potential of particles in a suspension is very helpful in controlling the addition of auxiliary agents to influence the flocculation and retention characteristics of particles. The addition of auxiliary agents can substantially influence the zeta potential and it is for this reason that much time has been devoted to methods and apparatuses to be used in determining the zeta potential of colloidal particles.
Many attempts have been made at measuring the zeta potential among them the use of ultrasonic sound to measure the vibration potential between two electrodes, and application of a laser beam for optical measurements thereof to determine the electrophoretic mobility of the migrating particles. The use of ultrasonic sound to measure the vibration potential of particles in suspension is disclosed in U.S. Pat. Nos. 4,294,656, 4,381,674 and 4,497,208.
U.S. Pat. No. 4,294,656 provides for a process for measuring zeta potential wherein a portion of the suspension is exposed to an ultrasonic field in a measuring cell, the measuring cell having two electrodes which extend into the suspension and are spaced from each other by an odd multiple of half ultrasonic wave length of the ultrasonic field in the suspension, and generating a signal from the voltage thereby formed between the electrodes which corresponds to the state of charge and determines the addition of auxiliary agent.
U.S. Pat. No. 4,381,674 discloses a method of detecting and identifying particulates in the recycling fluid flow of an oil recovery system by counting the number of ultrasonic pulses reflected from the particulates and comparing the number counted with the amount of ultrasonic energy across the flow.
U.S. Pat. No. 4,497,208 discloses a method and apparatus for measuring electro-kinetic properties of charged particles dispersed in a liquid medium which comprises the step of positioning two electrodes to contact the liquid medium, energizing the electrodes with an alternating electrical potential to cause a charged separation between the surfaces of the dispersed particles and the charged layers which surround the particles in the liquid medium and thereby to generate an acoustic signal, spacing an acoustic transducer from the electrodes for detecting an acoustic signal, and measuring the amplitude of the detected signal, the amplitude of the detected signal apparently being a function of the electro-kinetic properties of the particles present in the liquid medium, the number of particles per unit volume and the amplitude of the excitation potential on the electrodes.
Another means of measuring the zeta potential of colloidal particles is described in U.S. Pat. No. 4,046,667 which provides for a microelectrophoresis apparatus for measuring the zeta potential or electrophoretic mobility of particles suspended in a bulk medium, e.g. colloids suspended in a liquid. It further provides for the use of a light beam, microscope and objective lens system for physically determining the zeta potential of colloidal particles. The use of the microscope and light beam according to U.S. Pat. No. 4,046,667, however, requires a highly trained technician to physically determine the zeta potential. This system is inherently subject to human error and also requires prolonged analysis prior to each measurement.
The aforementioned patents relate either to the measuring of a vibration potential by use of ultrasonic sound in a frequency range of above 100 khz or to the measurement of electrophoretic mobility by the use of optical methods. Ultrasonic methods which measure a so-called "vibration potential" suffer from a lack of adequate theory linking vibration potential measurements to familiar colloidal properties such as zeta potential. Optical methods suffer from complexity and human error in the measurement of electrophoretic mobility.
The present inventor proposes a novel means and apparatus which avoids the inaccuracies and disadvantages of the prior art. Moreover, the present invention provides a method and apparatus for detecting an electrical signal which is directly proportional to the zeta potential of colloidal particles. The present invention also overcomes the disadvantage of the prior art which requires periodic testing by trained technicians. The advantages of a method and apparatus according to the present invention will be further described below.